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Plant the Right Corn Seed Population

What corn populations work best on your farm? Are you planting the right numbers? Knowing that answer can mean the difference between a so-so yield and a great one.

So, where's the bull's seye for your farm? The biggest variable, experts say, is moisture. Get a lot and you can plant higher populations. Take Nebraska, for example. There's about a 10,000-plant-per-acre difference between irrigated and dryland corn in that state, says University of Nebraska Extension soil specialist Charles Wortmann.

"Current Nebraska research suggests that yield increases may be possible by increasing plant population at harvest beyond the USDA-reported averages in Nebraska of 29,000 plants per acre (30,450 to 31,900 seeds per acre) for irrigated production and 21,850 plants per acre (22,943 to 24,035 seeds per acre) for rainfed production," Wortmann says in a university report, citing a recent publication on corn row spacing and seeding rates in that state. "Planting 34,000 seeds per acre for irrigated corn and 24,000 to 30,000 for rainfed corn, depending on expected yield, with 30-inch row spacing is expected on average to give the best net returns."

Corn plant populations – and recommendations like these – have for the last few years been a topic of a lot of talk in corn country. And, for quite a few reasons, says Purdue University Extension corn specialist Bob Nielsen.

"One of the reasons that the topic of seeding rates is a popular one in coffee shops, Internet chat rooms, the farm press, and crop seminars is that variable-rate seeding technology is becoming more and more commonplace today as a standard accessory on corn planters. Another factor that spurs the interest in corn seeding rates is the not uncommon belief that today's hybrids will respond dramatically to aggressively high plant populations," he says. "The harvest populations often associated with national corn yield contest winning entries, coffee shop scuttlebutt, and encouragement from seed company marketing efforts fuel this belief."

Genetics is the biggest reason for the upswing in plant populations; in the last 16 years alone, there's been a major shift. In Indiana, almost half of Indiana's corn acres were planted at a seeding rate lower than 25,000 per acre. In 2012, that number had fallen to 14%, while half of the state's acres were planted above 30,000 plants per acre.

"Among the changes that have allowed growers to steadily increase plant populations has been the genetic improvement in overall stress tolerance that has resulted in ear size and kernel weight becoming less sensitive to the stress of thicker stands of corn and improved late-season stalk health," Nielsen says, who says ultimately, under "normal growing conditions," his research shows optimal yield at a plant population of 31,400 plants per acre based on a percent stand of 95%. "That would translate to a seeding rate of 33,000 seeds per acre," he adds.

Though Nielsen says that 33,000 number may be optimal from an agronomic standpoint, that may not be true in an economic sense. And, when it comes to economically, there's always a "sweet spot" that takes both seed cost and grain market prices into account. For example, Nielsen says that at a $5-per-bushel corn price, $200-per-bag seed would dictate a plant population of just over 29,000 an acre to "maximize marginal return." At the same grain market price, $375-per-bag seed would be best planted at a 27,500-per-acre rate.

"Clearly, your market price for grain and the cost of seed influence the estimation of an economically optimum seeding rate," Nielsen says. "Recognize that percent stand establishment also influences the calculation of economic optimum seeding rates. If percent stand in your fields is historically lower than desired, then you either need to figure out how to improve the success of stand establishment or you need to use a higher seeding rate just to achieve the desired economically optimum final stand and, therefore, incur even more seeding cost."